Once the ticketing is done, I think there is a charge if you change route or airline. you maybe able to get that waive for being an exp. Out of the several SIN-HKG flight each day, only 1 has first class. also Cathay sometimes switch equipment. sometimes even when you have a F booking and they change equipment on the last minute, there is not much you can do. all you can do is check the equipment on the website, see if there is last min change maybe call AA to rebook youself into the one with F seat.

are you flying ORD-SFO-HKG-SIN-NRT-ORD